1. Field of the Invention
The present invention relates to a device for designing a standby path, a method for designing a standby path and a program therefor. In particular, it relates to designing a standby path used in on-fault switching, in which switching from a current path to a standby path is conducted when a fault occurs on a network.
2. Description of the Prior Art
For on-fault switching, in which switching from a current path to a standby path is conducted when a fault occurs on a network, the standby path may be designed to bypass the point of the fault when the fault occurs or may be designed in advance of occurrence of the fault.
In the latter case, in order to cope with a fault occurring at any point in the current path, the standby path needs to be designed not to pass through any node or link involved in the current path.
An example of such a conventional method of designing a standby path is described in Japanese Patent Laid-Open No. 7-250356. In this example, a current path is first searched for according to shortest route search. Then, shortest route search is performed for a standby path with lengths of optical transmission paths connected to the nodes that the current path has passed through being set to an adequately high value, so that the optical transmission paths involved in the current path are not chosen in shortest route search for the standby path.
In this way, the standby path is designed which does not involve any optical transmission paths involved in the current path. FIG. 16 shows a configuration of a conventional device for designing a standby path according to this example. In a device 2 for designing a standby path shown in FIG. 16, a current path search unit 21 first searches for a current path, and then, a standby path searching network creation unit 22 imparts an adequately long length to each of the optical transmission paths connected to the nodes that the current path passes through. Then, a standby path search unit 23 searches for a standby path according to shortest route search.
If the standby path is searched for after the current path is determined as described above, no standby path may sometimes be retrieved depending on a manner of setting up the current path. An example of such a case will be described with reference to FIGS. 4 and 5. In FIG. 4, reference symbols A, B, C, D, E and Z each denote nodes, and numeric characters appended to links connecting nodes together each denote lengths of the respective links. It is assumed that a current path and a standby path both extending from a start point (node A) to an end point (node Z) are to be designed. In FIG. 4, if the shortest path from the start point (node A) to the end point (node Z) is adopted as the current path, a current path (A-B-C-Z) results.
In order to retrieve a standby path that does not involve any link involved in the current path (A-B-C-Z), a path extending from the start point (node A) to the end point (node Z) is searched for in a network with a link between the nodes A and B (bidirectional), a link between the nodes B and C (bidirectional) and a link between the nodes C and Z (bidirectional) being excluded. However, as shown in FIG. 5, the network includes no route that reaches from the start point (node A) to the end point (node Z), and therefore, retrieval of a standby path results in failure.